Advantages:: Disadvantages
Advantages:: Disadvantages
Advantages:: Disadvantages
Advantages:
-An EDS system does not have moving parts such as the rotation detector with WDS.
-EDS systems are relatively faster.
-EDS System is low cost.
Disadvantages:
-The typical resolution of energy dispersion is about 150–200 eV, worse than the
corresponding resolution of WDS
-The lightest element that can be detected is O (Z = 8), not C (Z = 6).
Detector
The Si(Li) is the most commonly used detector in an
EDS system.
A spectrum in a range from 0.1 to about 10–20 keV can show both light and heavy
elements
The resolution, in terms of energy, is about 10 times lower than that of WDS. Even so,
EDS is an attractive technique for qualitative analysis of elements.
EDS spectrum of glass which includes Si, O, Ca, Al, Fe, and Ba
XRF Working Atmosphere and Sample Preparation
An XRF spectrometer can work in an internal atmosphere of air, helium, or
vacuum. However, detection sensitivity of fluorescent X-rays is highly affected
by the atmosphere.
EDS-type spectrometer can detect elements down to carbon (atomic number 6) in vacuum,
but limits its detection to sodium (atomic number 11) in helium, and to aluminum (atomic
number 13) in air.
Samples for examination in an XRF spectrometer can be bulk, powder, and liquid.
The basic requirements for solid samples are chemical homogeneity and physical flatness
of the face pointing towards the primary X-ray irradiation.
The samples are commonly contained in cup-type sample holders with a diameter
of 25–48 mm
The largest sample can be of a size of over 400 × 300 × 100 mm.
The primary X-ray irradiates an area of sample (∼5 cm2) from the bottom of the
sample holder.
X-ray penetration into a solid is limited to tens of micrometers from a solid surface even
though the thickness of a solid sample can be tens of millimeters.
A powder sample is ground and then pressed into pellets with high pressure.
A liquid solution sample can be directly examined in a special liquid sample holder with an
X-ray transparent bottom
Energy Dispersive Spectroscopy in Electron Microscope
The EDS type of X-ray spectrometer is commonly included as a part of scanning
electron microscopes (SEMs) and TEMs.
The main difference between EDS in an EM and in a standalone XRF is the source to excite
characteristic X-rays from a specimen. Instead of using the primary X-ray beam, a high-
energy electron beam (the same beam for image formation) is used by the X-ray
spectrometer in the microscopes
Special Features
EDS in the SEM is fundamentally similar to a standalone EDS except for the primary
beam source
In the SEM, the electron beam aligns with the vertical axis of the microscope so
that the Si(Li) detector has to be placed at a certain angle from the vertical
Figure. Potential interference of X-ray detection
due to low take-off angle in the SEM
Stationary mode:
-The probe stays at one location until the collection of X-ray photons is complete.
-The dwell time of the probe for EDS analysis is determined by the
number of X-ray photon counts received by the detector.
-The longer the dwell time, the higher the counts of characteristic X-ray photons (Low
concentration of trace elements (< 1 wt%) requires longer dwell times for their detection)
Scanning mode:
-The electron probe moves over the
specimen surface
-The intensity of specific characteristic X-
rays can be recorded and superimposed on
the corresponding electron image
Figure. EDS maps of a polished area of an alloy specimen. The distributions and concentrations of
chemical elements shown in the maps include: (a) Si; (b) Mo; (c) Cr; and (d) Co.
Stationary mode can be selected to analyze the elements in microscopic features such as
impurities, precipitates, and grain boundaries.
Scanning mode can be selected to examine the composition change in a certain dimension of
the specimen or composition variations in a selected area of a specimen.
C
Mg Si O
Ba Ca S